Hi,
I suspect that you are trying to set up the app charge for the store. As you are trying to add a test credit card, Shopify of course doesn’t accept it.
To try out the app charge in Shopify, simply mark the app charge as “test charge” through the API when you are creating it. This will allow you to accept it even in the development store.
These is a documentation about this here: https://shopify.dev/docs/api/admin-rest/2023-10/resources/recurringapplicationcharge
